This invention relates to wastewater treatment, and more particularly to an improved method of treating wastewater in a recirculating filter.
A septic tank system that includes secondary treatment of septic tank effluent prior to its on-site disposal provides treatment of wastewater in locations where soil conditions are not suitable for subsurface disposal of septic tank effluent. A recirculating sand filter is a popular choice for providing secondary treatment for the septic tank effluent because it is relatively inexpensive and moderately easy to retrofit, requires only semi-annual routine maintenance at moderate costs, and produces a fairly high quality effluent for on-site disposal.
In a conventional septic tank system including a recirculating sand filter, wastewater flows from a house to a septic tank for gravitational separation of solids from the liquid portion of the wastewater. The liquid portion of the wastewater is conducted from the septic tank to a recirculation tank where it mixes with filtrate returned by gravity from a sand filter. A pump controlled by a timer pumps the liquid from the recirculation tank into a distribution manifold in the sand filter. The liquid passes through the sand filter and is collected as filtrate. The filtrate is conducted through a conduit where, typically, a valve operates so that the liquid filtrate is either returned to the recirculation tank or it is discharged from the system for final disposal. During periods of zero flow of liquid wastewater from the septic tank to the recirculation tank, the valve typically operates to return all the sand filter filtrate to the recirculation tank to dilute the liquid already present there. When wastewater flows from the septic tank to the recirculation tank the liquid level in the recirculation tank rises. When the liquid in the recirculation tank reaches a predetermined level the valve operates so that none of the filtrate is returned to the recirculation tank. Instead, all the filtrate is discharged to disposal. However, the dilution of the liquid in the recirculation tank by the return of filtrate from the filter is necessary to maintain the biological state of the recirculation tank. In addition, during periods of heavy system usage, it is possible that liquid is circulated only once through the sand filter before being discharged for final disposal. Liquid which has passed only once through the sand filter has significantly poorer quality than liquid circulated more than once through the sand filter and will not meet the desired quality of the disposal system.
Orenco Systems, Inc. of Roseburg, Oreg. sells a septic tank system including a recirculating sand filter. In this system, the sand filter filtrate is directed to a flow splitter basin 1 (FIGS. 1 and 2). The basin 1, sized to match the flow capacity of the septic tank, typically has a diameter of 21 to 30 inches. Two orificed conduits 2, 3 located at the bottom of the basin act as drainage conduits, each conduit having a cap 4 at one end. The first conduit 2 is connected to the recirculation tank. The second conduit 3 is connected to a discharge line 9 for discharge of liquid from the system. An overflow pipe 8 is attached to each conduit 2, 3. An electrically actuated shut-off valve 5 is located on the discharge line 9. Sand filter filtrate enters the basin 1 through a conduit 6 and flows through the orifices 7 into the conduits and is conducted through the first conduit 2 to the recirculation tank and through the second conduit 3 to discharge from the system. The volume of flow through each conduit is determined by the relative number and size of the orifices in each conduit. The shut-off valve 5 on the discharge line 9 is actuated by a low level of liquid in the recirculation tank to eliminate flow of liquid through the second conduit to discharge. However, the flow splitter basin with orificed conduits cannot-be used without its associated energy-consuming electrical components.
Mayer U.S. Pat. No. 4,818,384 discloses a septic tank system with a recirculation tank receiving discharge from a septic tank and filtrate from a recirculating sand filter. A conduit conducting filtrate from the sand filter to the recirculation tank includes a float valve which closes during periods of heavy system usage and prevents any filtrate from entering the recirculation tank. However, the system cannot operate at peak efficiency under heavy usage conditions because none of the sand filter filtrate is available to dilute the liquid in the recirculation tank.
Halvorson U.S. Pat. No. 2,441,813 discloses a method and a system of treating wastewater by first removing settleable solids from the wastewater in a primary settling tank and transferring the liquid portion of the wastewater from the settling tank to a reservoir. A small portion of the liquid in the reservoir is continuously withdrawn and supplied to an aerobic trickling filter. Filtrate may be returned to the reservoir or diverted to a final settling tank prior to discharge from the system. However, the volume of liquid returned to the reservoir does not depend upon the amount of wastewater present in the reservoir.
Thus, there is still a need for a simple and economical system for treating sewage wastewater in a septic tank system that includes a recirculating sand filter which maintains the biological state in the recirculation tank under all flow conditions from the septic tank.